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small sequence zooms-in onto the Mira-like star T Leporis,
which was imaged with great detail, by ESO’s Very
Large Telescope Interferometer. T Leporis, in the
constellation of Lepus (the Hare), is located 500
light-years away. Mira stars are among the biggest factories
of molecules and dust in the Universe, and T Leporis is no
exception. It pulsates with a period of 380 days and loses
the equivalent of the Earth’s mass every year. Since
the molecules and dust are formed in the layers of
atmosphere surrounding the central star, astronomers would
like to be able to see these layers. But this is no easy
task, given that the stars themselves are so far away –
despite their huge intrinsic size, their apparent radius on
the sky can be just half a millionth that of the Sun. The
full resolving power of the VLTI is thereby required.Note
that the images are drawn to scale.

Credit:
ESO, Digitized Sky Survey 2 and A. FujiiA team
of French astronomers has captured one of the sharpest color
images ever made. They observed the star T Leporis, which
appears, on the sky, as small as a two-story house on the Moon.
The image was taken with ESO's Very Large Telescope
Interferometer (VLTI), emulating a virtual telescope about 100
meters across and reveals a spherical molecular shell around an
aged star.

“This
is one of the first images made using near-infrared
interferometry,”
says lead author Jean-Baptiste Le Bouquin. Interferometry is a
technique that combines the light from several telescopes,
resulting in a vision as sharp as that of a giant telescope with
a diameter equal to the largest separation between the telescopes
used. Achieving this requires the VLTI system components to be
positioned to an accuracy of a fraction of a micrometer over
about 100 meters and maintained so throughout the observations —
a formidable technical challenge.

When
doing interferometry, astronomers must often content themselves
with fringes, the characteristic pattern of dark and bright lines
produced when two beams of light combine, from which they can
model the physical properties of the object studied. But, if an
object is observed on several runs with different combinations
and configurations of telescopes, it is possible to put these
results together to reconstruct an image of the object. This is
what has now been done with ESO’s VLTI, using the 1.8-meter
Auxiliary Telescopes.

“We
were able to construct an amazing image, and reveal the
onion-like structure of the atmosphere of a giant star at a late
stage of its life for the first time,”
says
Antoine Mérand, member of the team. “Numerical
models and indirect data have allowed us to imagine the
appearance of the star before, but it is quite astounding that we
can now see it, and in color.”

Although
it is only 15 by 15 pixel across, the reconstructed image shows
an extreme close-up of a star 100 times larger than the Sun, a
diameter corresponding roughly to the distance between the Earth
and the Sun. This star is, in turn, surrounded by a sphere of
molecular gas, which is about three times as large again.

T
Leporis, in the constellation of Lepus (the Hare), is located 500
light-years away. It belongs to the family of Mira stars, well
known to amateur astronomers. These are giant variable stars that
have almost extinguished their nuclear fuel and are losing mass.
They are nearing the end of their lives as stars, and will soon
die, becoming white dwarfs. The Sun will become a Mira star in a
few billion years, engulfing the Earth in the dust and gas
expelled in its final throes.

Mira
stars are among the biggest factories of molecules and dust in
the Universe, and T Leporis is no exception. It pulsates with a
period of 380 days and loses the equivalent of the Earth’s
mass every year. Since the molecules and dust are formed in the
layers of atmosphere surrounding the central star, astronomers
would like to be able to see these layers. But this is no easy
task, given that the stars themselves are so far away —
despite their huge intrinsic size, their apparent radius on the
sky can be just half a millionth that of the Sun.

“T
Leporis looks so small from the Earth that only an
interferometric facility, such as the VLTI at Paranal, can take
an image of it. VLTI can resolve stars 15 times smaller than
those resolved by the Hubble Space Telescope,”
says Le Bouquin.

To
create this image with the VLTI astronomers had to observe the
star for several consecutive nights, using all the four movable
1.8-meter VLT Auxiliary Telescopes (ATs). The ATs were combined
in different groups of three, and were also moved to different
positions, creating more new interferometric configurations, so
that astronomers could emulate a virtual telescope approximately
100 meters across and build up an image.

“Obtaining
images like these was one of the main motivations for building
the Very Large Telescope Interferometer. We have now truly
entered the era of stellar imaging,”
says
Mérand.

A
perfect illustration of this is another VLTI image showing the
double star system Theta1 Orionis C in the Orion Nebula
Trapezium. This image, which was the first ever constructed from
VLTI data, separates clearly the two young, massive stars from
this system. The observations themselves have a spatial
resolution of about 2 milli-arcseconds. From these, and several
other observations, the team of astronomers, led by Stefan Kraus
and Gerd Weigelt from the Max-Planck Institute in Bonn, could
derive the properties of the orbit of this binary system,
including the total mass of the two stars (47 solar masses) and
their distance from us (1350 light-years).